It is known to make use of functional parts of a TV receiver to also receive FM radio signals, thereby allowing some commonality which reduces costs. Thus, for instance U.S. Pat. No. 6,633,345 B2 uses the same continuously tuneable TV tuner for both FM radio and TV audio by synthesising a sound subcarrier for the FM radio signals and using the normal TV demodulator.
U.S. patent application 2004/0088725 A1, by contrast, proposes either separate tuners for TV and FM but uses the same audio demodulator or extracts an FM radio signal from the tuner and provides, in an unexplained manner, the correct frequency to convert this to match the standard TV demodulator.
Such proposals fail to meet the requirements for easy production of the required output, firstly because the frequency separation of TV channels, which is programmed into the TV tuner, is not the same as the frequency separation of FM radio channels and secondly because the FM signal requires the generation of a picture frequency carrier if the same audio modules are to be used. Thus a TV tuner may have steps of 62.5 kHz while FM radio channels are normally 50 kHz apart. This provides difficulty in tuning accurately across the FM radio frequency band. Insertion of a picture carrier frequency to provide for use with the standard TV audio demodulator is also problematic without any picture frequencies. Existing methods of tuning FM radio signals therefore tend to spread the intermediate FM signals produced across the pass band of a TV tuner and can easily produce adjacent channel interference problems which are difficult to solve and require added componentry.
The present invention aims to provide a solution to this and other problems which offers advantages over the prior art.